In December 2012 a German knife manufacturer filed an injunction against Martha Stewart, Emeril Lagasse and the Home Shopping Network over a very unusual accusation. The knife maker, known as Solingen, claims that Stewart, Lagasse and HSN have all been selling counterfeit Solingen cutlery: knives with "Solingen Germany" stamped on one side and "China" on the other, to be precise.

This isn't a small-potatoes lawsuit. The fraud-detection agency Document Security Systems estimates that businesses lose $350 billion to counterfeiters every year, and Solingen is suing these three defendants for $2 millionper mention of the counterfeit knives. Multiply that by HSN's 24-hour, 365-day broadcast schedule to get a sense of the scale we're talking about here.

The good news is that a lot of customers quickly figured out that their knives were counterfeits, their first clue, presumably, being the word "China" stamped on the side. But this case raises some interesting questions: Without that stamp, how many customers would've figured out the difference -- and how long would it have taken them? For that matter, how do we recognize celebrities like Stewart and Lagasse as, well, themselves, and not doppelgängers out to deceive us?

The truth of the matter, the latest studies say, is that your brain's recognition system is neither simple nor always predictable.

Have you ever felt sure you'd seen your friend's face in a crowd only to realize that you'd just creeped out a stranger by staring too long? Then you've experienced something not too different from the Fregoli delusion, a psychological disorder characterized by a false belief that different people are in fact the same person in different outfits and hairstyles. Some recent studies have said that the delusion results from hyperexcitability in face-recognition brain areas, like the fusiform gyrus, which is the sort of explanation doctors trot out when they know where the problem is but aren't sure what it is.

At the opposite end of the spectrum is Capgras" delusion, a persistent belief that friends or family members are actually impostors intent on trickery. Freud himself was familiar with cases of this delusion, and he claimed -- as you can probably guess -- that when a patient's brain was rewired by trauma, it reverted to childhood instincts like sexual attraction for one's mother, which the patient's conscious mind proceeded to reject along with all feelings of warmth and recognition toward her. As limiting and mind-bending as this explanation was, it stuck around until neuroscientists came up with some better theories. Damage to the brain's face-recognition areas and the hyperexcitability of fear-related areas like the amygdala can both play important parts -- and one research team recently concluded that both are often involved, along with "inadequate evaluation of beliefs." I'm sure we could all name someone who suffers from that last symptom.

In short, your brain's recognition system works on several near-simultaneous levels: It matches people (or other objects) with memory data, it matches those memories with emotional responses, and if all goes well, it double-checks those responses for false positives. Even that may not prevent you from thinking that that guy or gal on the street is more attractive on the first glance than on the second; probably because missed opportunities are more dangerous than false positives when it comes to checking out potential mates.

And that instinct to avoid missing an opportunity, it would seem, is what drove all those customers to shell out cash for knives they'd never inspected in person. I know it's what drives my eBay purchases after I've had a bit too much Friday-night revelry.

This legal battle and these mental disorders -- and my eBay shopping history -- remind us of something we could all probably stand to keep in mind: In the end, all that stands in the way of delusion is a willingness to step back and double-check what we think we're seeing. Though we might not all be able to afford agencies like DSS to detect fraud for us, a lifelong habit of reality testing can serve essentially the same purpose. As the old folks in my family like to say, "If it seems too good to be true, it probably is."